Re: [PATCH rcu 2/6] rcu: Clarify rcu_is_watching() kernel-doc comment

[Joel Fernandes]

Hi Paul,

On 7/18/23 14:12, Paul E. McKenney wrote:
> On Tue, Jul 18, 2023 at 08:52:30AM -0400, Joel Fernandes wrote:
> > Hi Paul,
> >
> > On 7/17/23 14:03, Paul E. McKenney wrote:
> > > Make it clear that this function always returns either true or false
> > > without other planned failure modes.
> > >
> > > Reported-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > ---
> > >    kernel/rcu/tree.c | 12 ++++++++----
> > >    1 file changed, 8 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 1449cb69a0e0..fae9b4e29c93 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -679,10 +679,14 @@ static void rcu_disable_urgency_upon_qs(struct rcu_data *rdp)
> > >    /**
> > >     * rcu_is_watching - see if RCU thinks that the current CPU is not idle
> >
> > Would it be better to modify the 'not idle' to 'not idle from an RCU
> > viewpoint'? This matches the comments in ct_nmi_enter() as well.
>
> We have the "if RCU thinks that" earlier.
>
> But maybe something like this?
>
>   * rcu_is_watching - RCU read-side critical sections permitted on current CPU?

Yes, that's better.

> > >     *
> > > - * Return true if RCU is watching the running CPU, which means that this
> > > - * CPU can safely enter RCU read-side critical sections.  In other words,
> > > - * if the current CPU is not in its idle loop or is in an interrupt or
> > > - * NMI handler, return true.
> > > + * Return @true if RCU is watching the running CPU and @false otherwise.
> > > + * An @true return means that this CPU can safely enter RCU read-side
> > > + * critical sections.
> > > + *
> > > + * More specifically, if the current CPU is not deep within its idle
> > > + * loop, return @true.  Note that rcu_is_watching() will return @true if
> > > + * invoked from an interrupt or NMI handler, even if that interrupt or
> > > + * NMI interrupted the CPU while it was deep within its idle loop.
> >
> > But it is more than the idle loop, for ex. NOHZ_FULL CPUs with single task
> > running could be idle from RCU's viewpoint? Could that be clarified more?
>
> Perhaps something like this?
>
>   * Although calls to rcu_is_watching() from most parts of the kernel
>   * will return @true, there are important exceptions.  For example, if the
>   * current CPU is deep within its idle loop, in kernel entry/exit code,
>   * or offline, rcu_is_watching() will return @false.
>
> (Where nohz_full CPUs are covered by kernel entry/exit code.)

To me, "kernel exit" does not immediately make the nohz_full CPU case 
obvious. But yes, your suggestion is an improvement so we can go with 
that. :)

Also because we agree on the changes, for next revision:
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel